Is there a word for a personal or informal definition?

First, you say "If we're speaking English, the definition of a word should be its corresponding text in the dictionary." but that's not really true.

There is no 'official' English dictionary since there's no 'official' definition of the English language. The English language is defined by the community that speak it and evolves over time. Dictionary-makers are just well-meaning people with expertise in lexicography who do their best to give us useful tools that reflect usage. Dictionaries are helpful, and the respectable ones carry a lot of authority - but that authority is due to the painstaking research and work that went into them, not just the virtue of being called a 'dictionary'.

Each of us is free to make our own definitions that fit the context we're dealing with. "Definitions" are just explanations of a word's meaning using some paraphrase. What really matters is communication and being understood.

That all said - if you say "the definition of good C++ code", the use of the definite article means you are referring to a particular definition. It may be Webster's or Stroustrup's. If you say "my definition of good C++ code", then it's clear that you are referring to a personal definition which may not be found in a published book.

How about operational definition? The term is defined in terms of how it is being used/ or described in your report/presentation/paper.

You might say "My standard for good C++ code..."

Standards vary according to the situation and the requirements. Having said that, they should be consistent under similar conditions (or they wouldn't be standards). In your case, you may have good reasons to define your own standard.

The word you are looking for is "aphorism" derived from the Greek word meaning "definition". The modern cultural artifact we call a "dictionary" is based on one interpretation of how to define a word's meaning. The current approach is based primarily on a philosophy of use - that a word means whatever its users use it to mean. This leaves out much useful information, which could be based on the kind of tacit knowledge Polanyi called "personal knowledge" (Michal Polanyi, "Personal knowledge") A definition attempts to clarify a field of meaning. The questioner should be free to share his tacit and explicit knowledge of programming. The degree to which people will accept it as a definition may depend on the degree to which it identifies and clarifies the important features.